The key recharge process for Lower Tertiary Aquifer in the vicinity of Barwon Downs is recharge from rainfall. Previous studies have provided some estimates of groundwater recharge to the LTA; however these often incorporate little or no field data and provide a broad range of recharge estimates. The objective of this study was to estimate groundwater recharge to the LTA in the Barwon Downs region using independent techniques to improve the accuracy and confidence in the numerical model.
Given the considerable variability in the spatial and temporal distribution of recharge, it is best practice to apply multiple methods to reduce the uncertainty of recharge estimates. This study used two methods to estimate recharge using chemical tracers – the tritium method and the chloride mass balance method. These methods were selected as they use field data to characterise actual recharge to the aquifer, integrate unsaturated zone processes and are applicable over the time scales of interest.
To support the isotope and chloride based estimates of recharge, a one dimensional unsaturated zone model was also developed. This model was used to simulate recharge in a number of different soil profiles. The main advantage of the model is that it can provide more detailed estimates of the month to month and year to year variability than the temporally averaged figures from chemical tracers.
This assessment concluded that groundwater recharge rates to the outcropping LTA over the last 50 years are most likely equivalent to an average rate of 9% and 11% of annual rainfall. However, recharge in some areas may be as high as 26% of the annual average rainfall. Additionally, it was found that historical recharge rates over the last 100 to 1000s of years may be considerably lower, representing around 5% of the modern annual average rainfall.